A stronger team is necessary given the competition in the Champions League and Premier League. Arsenal must concentrate on having a strong bench team in addition to strengthening their starting group. Key player injuries are a real possibility and without a reliable backup the team performance as a whole could suffer significantly. Therefore in order to properly handle the demands of both tournaments Arteta and the club management must place a high priority on squad depth.
I would say Arsenal is a strong team as it is and is still capable of fighting for the title of the EPL. I agree with you on the need to concentrate on having an equally strong squad as substitutes for the starting eleven. Arsenal witnessed some setback when they had some key players injured and no one suitable enough to fill in the roles of the injured player. Without suitable replacements to fill in these roles when needed, the performance of the team would suffer setback as witnessed by Arsenal.
They would definitely need to make some drastic improvements as they’ve now qualified to be in the race for the title of the UCL. They would meet and play against other top clubs from other leagues.
However, Arsenal next season will not be strengthened by one of its flagship midfielders, Granit Xhaka, who will leave at the end of this season. It looks like Arteta will have to think about new player options to fill the position left by Xhaxa next season. We know that Arsenal's current squad is almost evenly distributed on various fronts, it's just that the options for second-tier players are still unbalanced, so it will be very difficult to compete with other teams in the EPL, especially Manchester City, which we know is Arsenal's arch rival this season. Even if Arsenal manage to qualify for the UCL, I think they will still have trouble focusing in several competitions and for sure their closest target will only be in the EPL race next season.